Dual exhaust was factory installed on GT equipped and 289 High Performance 65-66 Mustangs. It is, of course, a popular add on now. If you are interested in the details, you may want to know how to tell if a 1965 or 1966 Mustang came from the factory with dual exhaust.
One way to tell is to pull out the rear seat. Here, a convertible is pictured, but this detail appears the same on the Fastback and Coupe models also. Take a look at the arrows. They are pointing to a "reinforcement" piece of sheet metal that is not found on factory single exhaust models.
This is just one of those interesting details. For you guys and gals not into those types of details, I'll just say that it is not necessary to have these plates welded on your floor pan to add dual exhaust. It is no problem adding the factory hangers and plates in the same location on a Mustang that was not originally equipped that way.